js: make non-payload data more encoding-tolerant

• src/Helpers.js: As non-payload data might come in different encodings, a conversion has been introduced that worked in most cases. Data like the userid might come in different encodings, which we don't know of. For now, a try..catch returns the data as they are if the utf-8 decoding fails. Sometimes this yields the correct result, sometimes it may not work, but it won't stop the whole operation anymore.